Copper alloy having high resistance to oxidation for use in leads on semiconductor devices and clad material containing said alloy

ABSTRACT

A Cu alloy having high resistance to oxidation for use in leads on semiconductor devices is disclosed. The alloy consists essentially of 7-15 wt % Mn, 10-30 wt % Zn, 0.2-10 wt % Ni, 0.1-3 wt % Al, with the balance being Cu and incidental impurities. Also disclosed in a Cu alloy clad material wherein the substrate is made of Cu or Cu alloy having high electrical conductivity and good heat dissipation, and the cladding or partial cladding is composed of the Cu alloy having the composition specified above.

TECHNICAL FIELD

The present invention relates to a Cu alloy having high resistance tooxidation for use in leads on semiconductor devices, and a clad materialcontaining said alloy.

BACKGROUND OF THE INVENTION

Semiconductor devices such as ICs and LSIs are typically fabricated bythe following steps:

(a) prepare a lead material from a strip having a thickness of 0.1 to0.3 mm;

(b) stamp out a lead frame conforming to the shape of the semiconductordevice to be fabricated;

(c) apply high-purity Si or Ge semiconductor elements to selected areasof the lead frame by thermocompression with an electrically conductiveresin such as Ag paste or by diffusion through a plating of Au, Ag, Nior their alloys formed on one surface of the lead material and whichplating is heated to 350° C. or higher, or by soldering with a binaryeutectic alloy such as Au-Si or Au-Ge;

(d) connect the semiconductor elements to the lead frame by gold wires(this is a bonding step);

(e) enclose with a protective plastic or ceramic package thesemiconductor elements, gold wires and parts of the lead frame to whichthe semiconductor elements have been bonded;

(f) remove unnecessary parts from the lead frame to form discrete leads;and

(g) apply a soldering material to the legs of the leads to enableconnection of the semiconductor device to the substrate.

The material of which the leads on semiconductor devices are made has tomeet various requirements: (1) good stampability; (2) heat resistancehigh enough to avoid deformation and softening due to heat resultingfrom bonding leads to semiconductor devices; (3) good heat dissipationand electrical conductivity; and (4) strength high enough to avoidbreakage due to bending and other mechanical shocks that may beencountered during shipment of the semiconductor devices or theirassembling into electrical machines. In terms of characteristic values,the material is required to have a tensile strength of at least 28kg/mm², a Vickers hardness of at least 80, an elongation of at least 2%,an electrical conductivity (which is indicative of thermal dissipationor conductivity) of at least 2% IACS, and a softening point (a measureof thermal resistance) of at least 350° C. Conventional materials thatsatisfy these requirements are Cu alloys such as Fe-doped copper,phosphor bronze and Ag-doped copper.

These materials have high electrical conductivity and hence good heatdissipation, but their resistance to oxidation is so low that leads madeof them form an oxide film when they are fusion-bonded to semiconductordevices or when the latter are packed with plastics. This oxide filmmust be removed before the leads are coated with a soldering material orsoldered to the substrate of each semiconductor device. But in order toremove the oxide film, ordinary fluxes which have corrosive effects onthe semiconductor device should be replaced by non-corrosive fluxes.However, non-corrosive fluxes are very ineffective and are not capableof completely removing the oxide film. Therefore, almost allsemiconductor manufacturers depend on chemical treatments such ascleaning with acids to remove the oxide film. But this method has onefatal defect: it is very difficult to treat only the leg portion ofleads with acid solutions and this causes problems in regard to qualitycontrol, yield and cost.

SUMMARY OF THE INVENTION

We have made varius studies to find a material that satisfies therequirements met by the conventional materials for leads onsemiconductor devices and which also has high resistance to oxidationand will not form an oxide film when leads made of this material arefusion-bonded to the semiconductor device or when the latter is packagedwith plastics. As a result, we have found that this object can beaccomplished by a copper alloy which contains 7-15 wt% Mn, 10-30 wt% Zn,0.2-10 wt% Ni and 0.1-3 wt% Al, and optionally, 0.2 to 3 wt% of at leastone of Fe, Co and Sn, with the balance being copper and incidentalimpurities. The alloy satisfies all the requirements met by theconventional materials for leads on semiconductor devices and at thesame time, it has so high a resistance to oxidation that it forms aminimal amount of oxide film when leads made of this material arefusion-bonded to the semiconductor device or when the latter is packagedwith plastics. Therefore, no chemical treatment such as cleaning withacids is necessary before coating the lead with a solder in the finalstep of the fabrication of a semiconductor device, and the desiredsolder coat can be formed by simply using a non-corrosive flux.

We have also found that similar results can be obtained from a cladmaterial in which the substrate is made of pure copper or a copper alloysuch as phosphor bronze, Ag-, Cr-, Zr- or Fe-doped copper and thecladding or partial cladding (i.e. covering only the surface area thatis to be coated with solder) is made of a copper alloy having the abovedefined composition. In this clad material, the substrate ensures highelectrical conductivity (hence good heat dissipation) and the claddingprovides high resistance to oxidation.

DETAILED DESCRIPTION OF THE INVENTION

The present invention has been accomplished on the basis of thesefindings. The criticality of the composition of the respectivecomponents of the copper alloy used independently or as the cladding ishereunder described.

(a) Mn

Manganese increases the strength of the alloy and greatly inhibits theformation of an oxide film in the presence of zinc. if the manganesecontent is less than 7 wt%, its intended advantages are not obtained. Ifits content exceeds 15 wt%, difficulties arise in melting a mixture ofalloying elements, and the cold workability of the resulting alloy isreduced. Therefore, the manganese content is defined to be in the rangeof 7 to 15wt%.

(b) Zn

As already mentioned, zinc has the ability to inhibit the formation ofan oxide film in the presence of manganese. Furthermore, zinc increasesthe strength of the alloy and reduces casting defects because of itscapability of dehydrogenation during melting. If the zinc content isless than 10wt%, its intended advantages are not obtained. If itscontent exceeds 30wt%, a great improvement in strength is obtained buton the other hand, the resulting alloy is very low in ductility and isnot highly adaptive to working such as rolling. Therefore, the contentof zinc is defined to be in the range of 10 to 30 wt%.

(c) Ni

Nickel is an element that provides increased strength and heatresistance. If its content is less than 0.2 wt%, this advantage is notobtained, and if its content exceeds 10 wt%, the resulting alloy is nothighly adaptive to working such as rolling. Therefore, the content ofnickel is defined to be in the range of 0.2 to 10 wt%.

(d) Al

Aluminum is effective for significant improvement of the strength andheat resistance of alloys. It also works as an effective deoxidizerduring melting and helps produce a sound casting. However, if thealuminum content is less than 0.1 wt%, its desired effects are notobtained, and if its content exceeds 3 wt%, the resulting alloy is nothighly adaptive to working such as rolling. Therefore, the aluminumcontent is defined to be in the range of 0.1 to 3 wt%.

(e) Fe, Co and Sn

These elements are effective in achieving a further increase in thealloy strength, and may be incorporated if higher strength is necessary.However, this advantage is not obtained if their content is less than0.2 wt%. On the other hand, if the content of Fe, Co or Sn exceeds 3wt%, the resulting alloy is not highly adaptive to processing such asrolling. Therefore, their content is defined to be in the range of 0.2to 3 wt%.

The copper alloy of the present invention and the clad material usingthat alloy are hereunder described in greater detail by reference to twoworking examples.

EXAMPLE 1

Melts having the compositions shown in Table 1 were prepared in agraphite crucible in atmophere placed in the conventional high-frequencyheating furnace. The melts were cast into slabs (40 mm×200 mm×3,000 mm)by the conventional continuous casting technique. After grinding thesurface, the slabs were hot-rolled into sheets 10 mm thick. The sheetswere subjected to repeated cycles of treatment consisting of coldrunning and bright annealing. The so treated sheets were slit intonineteen semi-hardened samples 10 mm wide and 0.25 mm thick. Sixteen ofthem were samples of the present invention, and the remainder weresamples of the three materials (Fe-doped copper, phosphor bronze andAg-doped copper) conventionally used in leads on semiconductor devices.

The individual samples were checked for their tensile strength, Vickershardness, elongation, electrical conductivity coefficient of thermalexpansion and softening point. In addition, the samples were held at250° C. (the temperature at which conventional semiconductors arepackaged with plastics) in atmosphere for 10, 20 and 30 minutes andthereafter checked for their wettability with solder using rosin as anon-corrosive flux. The results are shown in Table 1, wherein O refersto good adhesion to solder and X refers to incomplete adhesion.

EXAMPLE 2

Cu and Cu alloy melts for the substrate and cladding having thecompositions indicated in Table 2 were prepared in a graphite cruciblein atmosphere placed in the conventional high-frequency heating furnace.The melts were cast into slabs (40 mm×200 mm×3,000 mm) by theconventional continuous casting technique. After grinding the surface,the slabs were hot-rolled into sheets 10 mm thick. The sheets weresubjected to repeated cycles of treatment consisting of cold rolling andbright annealing. By assembling these cold-rolled sheets according tothe combinations indicated in Table 2, sixteen semi-hardened samples ofclad material measuring 30 mm wide and 0.25 mm thick and having theconstructions shown in Table 2 (A: the substrate was sandwiched by acladding 0.025 mm thick; B: the substrate was clad on one side with alayer 0.03 mm thick; C: a cladding 7.5 mm wide and 0.05 mm thick wasformed along two side bands on one surface of the substrate; D: acladding 7.5 mm wide and 0.05 mm thick was formed along two side bandson both surfaces of the substrate; E: the substrate was clad on one sidewith a layer 0.05 mm thick; F: the substrate was sandwiched by acladding 0.05mm thick) were prepared.

The individual clad samples were checked for their tensile strength,elongation and electrical conductivity. In addition, the Vickershardness of the substrate and cladding of each sample was measured. Theresults are shown in Table 2. As in Example 1, the clad samples wereheld at 250° C. in atmosphere for 10, 20 and 30 minutes and thereafterchecked for their wettability with solder using rosin as a non-corrosiveflux. The results are also shown in Table 2, wherein O refers tocomplete adhesion to solder and X refers to incomplete adhesion.

For purposes of comparison, the substrates, or the conventional Cu alloysamples, were subjected to the same measurements and the results areshown in Table 2.

                                      TABLE 1                                     __________________________________________________________________________                                                  Coeffi-                                                                       cient of                                                                              Wettability                                                     Elec- thermal with solder                                                     trical                                                                              expan-                                                                            Soft-                                                                             10-                                                                              20-                                                                              30-                                        Tensile                                                                             Elonga-                                                                            Hard-                                                                             conduc-                                                                             sion                                                                              ening                                                                             min.                                                                             min.                                                                             min.              Cu Alloy                                                                            Composition (wt %) Strength                                                                            tion ness                                                                              tivity                                                                              (10.sup.-6 /                                                                      paint                                                                             heat-                                                                            heat-                                                                            heat-             samples                                                                             Mn Zn Ni                                                                              Al                                                                              Fe                                                                              Co                                                                              Sn                                                                              Cu (kg/mm.sup.2)                                                                       (%)  (Hv)                                                                              (% IACS)                                                                            °C.)                                                                       (°C.)                                                                      ing                                                                              ing                                                                              ing               __________________________________________________________________________    Samples of                                                                    the present                                                                   invention                                                                      1    7.3                                                                              29.3                                                                             3.1                                                                             0.9                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             51    2.9  135 2.8   17.0                                                                              625 O  O  O                  2    10.2                                                                             21.0                                                                             3.0                                                                             0.8                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             52    3.0  135 3.0   17.0                                                                              620 O  O  O                  3    14.5                                                                             10.8                                                                             3.3                                                                             1.1                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             50    2.5  130 3.1   17.5                                                                              630 O  O  O                  4    10.1                                                                             24.9                                                                             0.3                                                                             1.0                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             47    2.3  130 2.8   17.5                                                                              620 O  O  O                  5    9.8                                                                              25.3                                                                             9.8                                                                             1.1                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             63    2.0  155 2.7   17.0                                                                              680 O  O  O                  6    14.2                                                                             25.0                                                                             2.0                                                                             0.2                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             55    2.1  140 2.8   17.5                                                                              635 O  O  O                  7    10.4                                                                             25.0                                                                             3.0                                                                             2.8                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             58    2.1  145 2.6   17.0                                                                              660 O  O  O                  8    10.1                                                                             25.2                                                                             5.3                                                                             0.8                                                                             0.3                                                                             --                                                                              --                                                                              bal.                                                                             56    2.3  142 2.7   17.1                                                                              663 O  O  O                  9    9.8                                                                              24.7                                                                             5.0                                                                             0.9                                                                             1.0                                                                             0.4                                                                             --                                                                              bal.                                                                             58    2.2  145 2.5   17.1                                                                              680 O  O  O                 10    10.3                                                                             24.9                                                                             4.3                                                                             0.7                                                                             2.5                                                                             --                                                                              0.2                                                                             bal.                                                                             59    2.0  149 2.7   17.0                                                                              673 O  O  O                 11    10.2                                                                             25.1                                                                             4.1                                                                             0.7                                                                             0.5                                                                             1.1                                                                             0.9                                                                             bal.                                                                             63    2.1  155 2.8   17.3                                                                              682 O  O  O                 12    9.7                                                                              24.8                                                                             4.5                                                                             0.8                                                                             --                                                                              2.8                                                                             --                                                                              bal.                                                                             62    2.2  153 2.7   17.2                                                                              680 O  O  O                 13    10.0                                                                             25.0                                                                             4.4                                                                             0.8                                                                             --                                                                              0.6                                                                             2.0                                                                             bal.                                                                             64    2.1  155 2.9   17.3                                                                              670 O  O  O                 14    13.8                                                                             18.2                                                                             3.1                                                                             1.0                                                                             1.1                                                                             --                                                                              --                                                                              bal.                                                                             59    2.3  149 2.9   17.1                                                                              673 O  O  O                 15    10.0                                                                             11.0                                                                             4.0                                                                             0.3                                                                             1.0                                                                             --                                                                              --                                                                              bal.                                                                             55    2.5  140 3.0   17.5                                                                              660 O  O  O                 16    10.1                                                                             25.5                                                                             3.0                                                                             0.7                                                                             --                                                                              --                                                                              1.0                                                                             bal.                                                                             55    2.6  141 2.9   17.8                                                                              660 O  O  O                 Conven-                                                                       tional                                                                        samples                                                                        1    Cu - 0.1% Zn - 2.4% Fe - 0.03% P                                                                 41    2.9  120 65.0  16.3                                                                              550 X  -- --                 2    Cu - 6% Sn - 0.2% P                                                                              65    3.5  150 17.0  16.5                                                                              350 X  -- --                 3    Cu - 0.1% Ag       28    3.0   82 98.0  17.7                                                                              350 X  -- --                __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Sub-                                             Elec-                                                                               Wettability            strate                                           trical                                                                             with solder             Cu  com-                        Tensile                                                                            Elon-                                                                             Hardness                                                                              conduc-                                                                            10-                                                                               20-                                                                             30-               clad                                                                              posi-                   Con-                                                                              strength                                                                           ga- (Hv)    tivity                                                                             min.                                                                             min.                                                                             min.              sam-                                                                              tion                                                                              Cladding compositon (wt %)                                                                        struc-                                                                            (kg/  tion                                                                             Sub-                                                                              Clad-                                                                             (%   heat-                                                                            heat-                                                                            heat-             ples                                                                              (wt %)                                                                            Mn Zn Ni Al                                                                              Fe                                                                              Co                                                                              Sn                                                                              Cu tion                                                                              mm.sup.2)                                                                          (%) strate                                                                            ding                                                                              IACS)                                                                              ing                                                                              ing                                                                              ing               __________________________________________________________________________    Sam-                                                                          ples of                                                                       the                                                                           present                                                                       inven-                                                                        tion                                                                          1   Zn: 0.1,                                                                          7.4                                                                              29.6                                                                             3.0                                                                              0.9                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             A   43   2.8 121 133 52.5 O  O  O                 2   Fe: 2.4,                                                                          10.1                                                                             21.2                                                                             3.1                                                                              0.8                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             A   43   2.9 122 134 52.6 O  O  O                 3   P: 0.03,                                                                          14.5                                                                             10.9                                                                             3.2                                                                              1.1                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             A   42   2.6 119 131 52.6 O  O  O                     Cu: bal.                                                                      (Fe-                                                                          doped)                                                                    4   Sn: 6,                                                                            10.0                                                                             25.1                                                                             0.3                                                                              1.0                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             B   63   3.3 150 133 15.3 O  O  O                 5   P: 0.2,                                                                           9.8                                                                              25.7                                                                             10.1                                                                             1.2                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             B   64   3.0 151 155 15.2 O  O  O                 6   Cu: bal.                                                                          14.4                                                                             24.8                                                                             2.0                                                                              0.2                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             B   63   3.1 150 143 15.3 O  O  O                     (phos-                                                                        phor                                                                          bronze)                                                                   7   Ag: 0.1,                                                                          10.3                                                                             25.0                                                                             3.0                                                                              2.8                                                                             --                                                                              --                                                                              --                                                                              bal.                                                                             C   31   2.6  83 141 88.5 O  O  O                 8   Cu: bal.                                                                          10.2                                                                             25.1                                                                             5.3                                                                              0.8                                                                             0.3                                                                             --                                                                              --                                                                              bal.                                                                             C   30   2.8  84 140 88.5 O  O  O                 9   (Ag-                                                                              9.8                                                                              25.0                                                                             5.1                                                                              0.9                                                                             0.8                                                                             0.5                                                                             --                                                                              bal.                                                                             C   31   2.8  83 142 88.4 O  O  O                     doped)                                                                    10  Cr: 0.8,                                                                          10.6                                                                             24.8                                                                             4.3                                                                              0.7                                                                             2.4                                                                             --                                                                              0.3                                                                             bal.                                                                             D   50   2.2 135 148 68.5 O  O  O                 11  Cu: bal.                                                                          10.0                                                                             25.2                                                                             4.0                                                                              0.7                                                                             0.5                                                                             1.1                                                                             0.8                                                                             bal.                                                                             D   51   2.1 132 154 68.6 O  O  O                     (Cr-                                                                          doped)                                                                    12  Zr: 0.5                                                                           9.8                                                                              24.6                                                                             4.4                                                                              0.8                                                                             --                                                                              2.8                                                                             --                                                                              bal.                                                                             E   48   2.2 105 153 69.3 O  O  O                 13  Cu: bal.                                                                          10.1                                                                             24.9                                                                             4.5                                                                              0.7                                                                             --                                                                              0.6                                                                             2.1                                                                             bal.                                                                             E   49   2.2 107 154 69.5 O  O  O                     (Zr-                                                                          doped)                                                                    14  Oxy-                                                                              13.9                                                                             18.0                                                                             3.2                                                                              1.0                                                                             1.2                                                                             --                                                                              --                                                                              bal.                                                                             F   40   3.6  81 148 61.2 O  O  O                 15  gen 9.9                                                                              11.2                                                                             3.9                                                                              0.3                                                                             1.1                                                                             --                                                                              --                                                                              bal.                                                                             F   38   3.5  81 140 61.3 O  O  O                 16  free                                                                              9.7                                                                              24.8                                                                             3.1                                                                              0.7                                                                             --                                                                              --                                                                              0.9                                                                             bal.                                                                             F   38   3.5  80 142 61.2 O  O  O                     Cu)                                                                           (pure                                                                         Cu)                                                                       Con-                                                                          ven-                                                                          tional                                                                        Sam-                                                                          ples                                                                          1   Zn: 0.1,                                                                          --                      39.1 3.1 118 --  65.0 X  X  X                     Fe: 2.4,                                                                      P: 0.03,                                                                      Cu: bal.                                                                      (Fe-                                                                          doped)                                                                    2   Sn: 6,                                                                            --                      44.6 2.8 149 --  15.1 X  X  X                     P: 0.2,                                                                       Cu: bal.                                                                      (phos-                                                                        phor                                                                          bronze)                                                                   3   Ag: 0.1,                                                                          --                      26.9 3.6  83 --  98.0 X  X  X                     Cu: bal.                                                                      (Ag-                                                                          doped)                                                                    4   Cr: 0.8,                                                                          --                      50.1 2.9 135 --  86.2 X  X  X                     Cu: bal.                                                                      (Cr-                                                                          doped)                                                                    __________________________________________________________________________

Table 1 shows that the copper alloy samples Nos. 1 to 16 of the presentinvention have good properties of strength, electrical conductivity andheat resistance. Furthermore, they exhibited good wettability to soldereven after they were exposed to 250° C. in atmosphere for 30 minutes.However, the three conventional samples did not have high resistance tooxidation, so when they were held at 250° C. in atmosphere for 10minutes, a thick oxide film formed and reduced their wettability withsolder.

Table 2 shows that the Cu alloy clad samples 1 to 16 of the presentinvention had a tensile strength of 28 kg/mm² or more, a Vickershardness of 80 or more, an elongation of 2% or more, and an electricalconductivity of 2% IACS or more. In addition, they were highly resistantto oxidation and exhibited good wettability with solder. On the otherhand, the conventional Cu alloy samples 1 to 4 were low in resistance tooxidation and had a very poor wettability with solder.

Because of these advantages, the Cu alloy of the present invention canbe effectively used as the material for leads on semiconductors, and theleg portion of the lead made of this alloy can be coated with solder orcan be soldered to the substrate of a semiconductor by simply using anon-corrosive flux without the need for a cumbersome chemical treatment.The same results are obtained with a clad material which has anoxidation-resistant cladding in at least the area to be soldered.

What is claimed is:
 1. A Cu alloy clad material for use in leads onsemiconductor devices comprising a substrate wholly or partially coveredby a clad metal, wherein the substrate is a Cu or Cu alloy having highelectrical conductivity and good heat dissipation, and the clad metal isa Cu alloy having high resistance to oxidation which consistsessentially of 7-15% Mn, 10-30% Zn, 0.2-10% Ni, 0.1-3% Al, thepercentage being by weight, with the balance being Cu and incidentalimpurities.
 2. A Cu alloy clad material for use in leads onsemiconductor devices comprising a substrate wholly or partially coveredby a clad metal, wherein the substrate is a Cu or Cu alloy having highelectrical conductivity and good heat dissipation, and the clad metal isa Cu alloy having high resistance to oxidation which consistsessentially of 7-15% Mn, 10-30% Zn, 0.2-10% Ni, 0.1-3% Al and 0.2-3% ofat least one of Fe, Co and Sn, the percentage being by weight, with thebalance being Cu and incidental impurities.